Bicycles are commonly provided with a series of parallel cogs/sprockets of varying diameter/tooth-counts fixed to the rear wheel of the bicycle concentric to the wheel axis, also considered a rear gear cassette. The cogs are typically arranged in a cone-like shape from small gear to large gear. A bicycle rider transfers power via the cranks (having a crank axis) in which a front chain ring is fixed. The front chain ring may include more than one cog/sprocket having differing sizes (e.g. also forming a cone) and be considered a front gear cassette. A drive chain/belt travels over the chain/belt ring to one of the rear cogs in a closed loop, driving the rear wheel.
The gear ratio between the front chain ring (power input) and rear wheel (power output) is determined by which rear cog the drive chain/belt has engaged. An example of a prior art rear derailleur used to shift the chain/belt is disclosed in FIG. 1. The rear derailleur 10 is a linkage mechanism that controls the position of the drive chain/belt 5 relative to individual cogs/sprockets 6 of the rear wheel. Currently, a linkage used in a rear derailleur 10 such as those commonly used today is a 2-dimensional planar 4-bar linkage 11 having a parallelogram structure. The resultant path of the floating link 20 of this mechanism is non-linear, forming a curved or arcuate path. As a result, the angle of the derailleur pulley axes 17, 19 are not constant relative to the wheel axis 7 in at least one reference plane throughout the entire travel range, which can create undesirable forces and negatively affect performance and wear and tear on the components. The wheel axis 7 is the axis defined by the rotation of the wheel hub 2.
Typical rear derailleurs 10 have of a parallelogram linkage 11 as shown in FIG. 1. One link, the stationary link 14, is fixed or pivotally mounted at connection 12 to the rear derailleur hanger 3 of the bicycle rear triangle 1 or swingarm, or to the rear triangle/swingarm itself. Two parallel links 16, 18 connect the floating link 20 to the stationary link 14. An actuation force is applied to change the position of the mechanism, typically via a cable. A return spring is connected to the parallelogram providing a force opposite to that of the actuation force. A derailleur cage assembly 30 is pivotably connected to the floating link 20. The derailleur cage assembly includes an upper pulley or jockey pulley 31, and a lower pulley or idler pulley 32.
Currently the most common linkage design used in rear derailleurs today is a planar 4-bar linkage parallelogram. There are several disadvantages to this mechanism. For example, the resultant path this linkage defines is non-linear curved. As a result, the angular relationship of the derailleur pulley axes 17, 19 varies with respect to the wheel axis 7 throughout the range of motion. The inherent geometry of the parallelogram leaves little freedom of the linkage mounting location relative to the rear wheel to achieve the desired linkage path. This limited freedom correspondingly limits frame designers options, whereas more freedom of this mounting location would give frame designer more options.
As noted previously, derailleur linkages 11 are activated via an actuation force to move the mechanism through its travel. Moving the mechanism through its travel causes the chain 5 to shift from one wheel cog in the cassette 6 to another wheel cog. One end of the actuation cable is connected to one of the non-stationary links 20 and the other to the stationary link 14 or bike frame itself. With a parallelogram design, the mechanism's linkage path is dependent upon the link lengths and axes geometry. In order to achieve an optimum linkage path and actuation ratio in a parallelogram mechanism, it is common to add additional complex features such as pulley wheels and extended links. These items add weight and complexity.
The inherent geometry of the parallelogram leaves little freedom to minimize the mechanism's volume envelope and envelope position relative to the drive side frame dropout. It is desirable to have a compact mechanism located as inboard as possible to the frame to minimize the chance of hitting the derailleur on an obstacle while riding, which can prove difficult to achieve with this design.
A mechanism that offers various solutions to the inherent mechanical limitations of a parallelogram design discussed above is desired.